Flyback converters are used as battery chargers and AC adapters that can supply a constant current to a heavy load and a constant voltage to a light load, for example as a battery becomes fully charged. Flyback converters can operate in discontinuous conduction mode (DCM), critical conduction mode (CRM) or continuous conduction mode (CCM). In discontinuous conduction mode, there is a time gap after all of the energy that was transferred to a secondary inductor has been released and before current again begins to ramp up through a primary inductor. In continuous conduction mode, current begins to ramp up in the primary inductor before current has stopped flowing through the secondary inductor to release the transferred energy. In critical conduction mode, current begins to ramp up in the primary inductor at approximately the same time as current stops flowing through the secondary inductor.
Operating a flyback converter in critical conduction mode has various advantages. For example, switching losses are reduced by the self-oscillating ability of flyback converters that operate in critical conduction mode. In addition, the peak inductor current required to achieve a given charging power is lower in critical conduction mode than in discontinuous conduction mode. Although the average current in both cases is the same, there is no dead time in critical conduction mode. Higher efficiency in energy transfer can be achieved with the lower peak current. In addition, higher efficiency is achieved in critical conduction mode than in continuous conduction mode in which the primary inductor begins charging before all of the energy has been released from the secondary inductor.
There are various existing designs for flyback converters that are self-oscillating and operate in critical conduction mode. FIG. 1 (prior art) illustrates an exemplary prior art self-oscillating flyback converter 10, also called a “ringing choke converter”. The self-oscillation is accomplished using two bipolar transistors. A first transistor Q1 11 acts as a switch to a primary inductor 12. A second transistor Q2 13 turns off first transistor Q1 11 at the end of each switching cycle. Converter 10 controls its output current and voltage by receiving feedback from the secondary side of a transformer 14 via an optical coupler 15. Converter 10 also uses a shunt reference U1 16 on the secondary side. For example, shunt reference U1 16 is a 3-pin part TL431 that regulates its third terminal to 1.25 volts.
Converter 10 has the disadvantage that it is relatively expensive because it takes many components to implement constant output voltage and constant output current controls. As shown in FIG. 1, shunt reference U1 16, resistors R2, R3 and R6 and capacitor C2 and are used for constant output voltage control. Transistor Q3 and resistors R4 and R5 are used for constant output current control. Optical coupler 15 is also required to transfer the control signal from the secondary side to the primary side of the flyback converter 10. The two external bipolar transistors 11 and 13 on the primary side, as well as optical coupler 15 and shunt reference 16, all add to the cost of converter 10. In addition, the discrete components of converter 10 are not as reliable over time compared to equivalent components in an integrated circuit.
When converter 10 is first turned on, current flowing through a start resistor RSTART 17 increases the base voltage VB of transistor Q111 and turns on transistor Q1 11. As transistor Q1 11 turns on, a positive regenerative feedback develops through an auxiliary inductor 18 and is applied through an R/C network 19 to the base of transistor Q1 11, turning on transistor Q1 11 rapidly. As the emitter current of transistor Q1 11 increases, the voltage across an emitter resistor RE 20 adds to a feedback voltage VFB from optical coupler 15 and increases the base voltage of transistor Q2 13. When transistor Q2 13 turns on, current is drained from the base of transistor Q1 11, and the base voltage VB decreases. As transistor Q1 11 begins to turn off, current stops flowing through primary inductor 12 and the voltages across all inductors of transformer 14 reverse in accordance with normal fly-back action. A regenerative turn-off results from current flowing through auxiliary inductor 18 and R/C network 19 to the base of transistor Q1 11. Transistor Q1 11 remains off until all of the energy that was stored in transformer 14 is transferred to the secondary side. In a heavy load condition, the next switching cycle to turn on transistor Q1 11 will be immediately kicked off by resonant current from auxiliary winding controlled by the feedback loop. In a light or no load condition, when current is no longer flowing through any of the inductors, the voltages across the inductors fall to zero. When the voltage across auxiliary inductor 18 is zero and input current through start resistor RSTART 17 has again accumulated on the base of transistor Q1 11, transistor Q1 11 turns on and a new cycle begins.
FIG. 2 (prior art) shows a much simpler implementation of a self-oscillating flyback converter 21. Converter 21 accomplishes self-oscillation using two bipolar transistors. Feedback to regulate output current and voltage is received from a reflected voltage through a transformer 22. Although converter 21 does not employ an optical coupler and a shunt reference, the two bipolar transistors and the many discrete components add to the cost of the convert.
Similar to converter 10, converter 21 also uses regenerative feedback to turn its transistors on and off. When converter 21 is first turned on, current flowing through a start resistor RSTART 23 increases the base voltage of a transistor Q1 24, and transistor Q1 24 begins to turn on. Transistor Q1 24 then turns on rapidly because, as it turns on, a current begins to flow through an auxiliary inductor 25. A positive regenerative feedback voltage develops across auxiliary inductor 25 and is applied through an R/C network 26 to the base of transistor Q1 24. As the emitter current of transistor Q1 24 increases, the voltage across an emitter resistor RE 27 increases the base voltage of a second transistor Q2 28. While transistor Q1 24 was off, the base voltage of second transistor Q2 28 was set by the feedback voltage across auxiliary inductor 25 as conditioned by a feedback network 29 of diodes D1 and D2, resistors R1 and R2 and capacitor C1. When the voltage across emitter resistor RE 27 increases past the voltage set by feedback network 29, transistor Q1 24 begins to turn off. In accordance with normal fly-back action, the voltages across all inductors of transformer 22 reverse when current stops flowing through a primary inductor 30 of transformer 22.
Now regenerative turn-off results from current flowing through auxiliary inductor 25 and R/C network 26 to the base of transistor Q1 24 and turns off transistor Q1 24 quickly. Transistor Q1 24 remains off until all of the energy that was stored in transformer 22 is transferred to the secondary side. In a heavy load condition, the next switching cycle to turn on transistor Q1 11 will be immediately kicked off by resonant current from auxiliary inductor 25 controlled by the feedback loop. In a light or no load condition, when current is no longer flowing through any of the inductors, the voltages across the inductors falls to zero. When the voltage across auxiliary inductor 25 is zero and input current through start resistor RSTART 23 has again accumulated on the base of transistor Q1 24, transistor Q1 24 begins to turn on and a new cycle begins.
Feedback network 29, which includes diodes D1 and D2, resistor R1 and capacitor C1, is used to control constant output voltage by turning off transistor Q1 24 SO as to vary its on time. The emitter resistor RE 27, resistor R2 31, capacitor C2 32 and second transistor Q2 28 are used to control constant output current. The main disadvantage of flyback converter 21 is the poor accuracy of the output voltage and the output current.
A less expensive self-oscillating, primary-side controlled flyback converter is sought that has fewer external components and that can operate in critical conduction mode but yet that accurately controls constant output current and voltage. For example, a self-oscillating, primary-side controlled flyback converter with only one external transistor is sought that is controlled by a controller IC contained in an IC package with few pins.